1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device having an over current protection circuit for protecting output transistors from an over current.
2. Description of the Related Art
In automobiles and home electric equipment, power devices (also called power ICs or power semiconductors), for controlling heavy current and high voltages, are being used. Power devices have the function of protecting output transistors from an over current, since an abnormal heavy current (over current) which flows into the output transistors due to such a failure as a load short circuit may breakdown the output transistors.
FIG. 14 and FIG. 15 are circuit diagrams depicting a configuration of a conventional power device. This conventional power device 101 is a switch for controlling the current which flows through the load 102, and this switch uses an MOSFET (Metal Oxide Semiconductor Field Effect Transistor, hereafter called MOS transistor). Since the switch is installed at a power supply (battery) side rather than a load, this is called a high side switch.
As FIG. 14 and FIG. 15 show, the conventional power device 101 comprises an output MOS transistor M110 for controlling the current of the load 102, a charge pump 103 for generating the gate voltage for turning ON the output MOS transistor M110, and the over current protection circuit 104 for protecting the output MOS transistor M110 from over current. The over current protection circuit 104 further comprises a detection circuit 105 for detecting an over current, and an MOS transistor M111 for discharging the gate charges of the output MOS transistor M110 when the detection circuit 105 detects an over current.
The battery to be the power supply is connected to the Vcc terminal (power supply terminal), the ground voltage GND is connected to the GND terminal (ground terminal), and the load 102 is connected to the OUT terminal.
For example, the output of the charge pump 103 is turned ON/OFF by a control signal from the outside, and the output MOS transistor M110 is turned ON/OFF by the output signal of the charge pump 103. When an over current is detected, the detection circuit 105 discharges the gate charges of the output MOS transistor M110 by turning ON the MOS transistor M111, and turns OFF the output MOS transistor M110, so as to prevent the output MOS transistor M110 from breakdown by an over current.
The power device 101 in FIG. 14 and the power device 101 in FIG. 15 are examples where the connection destination of the source of the MOS transistor M111 is different. The source of the MOS transistor M111 in FIG. 14 is connected to the GND terminal (ground terminal), and the source of the MOS transistor M111 in FIG. 15 is connected to the OUT terminal (output terminal). When the load is shorted, the OUT terminal is grounded to the ground voltage GND, so the power devices in FIG. 14 and FIG. 15 perform the same operation.
FIG. 16 is a timing chart when the load is shorted in a conventional power device. A short of the load means that the OUT terminal is grounded to the ground voltage GND, without passing through the load, by such a reason as a stripped insulating coating and a loose connector. If the load is shorted, the power supply voltage Vcc at the Vcc terminal drops to a level close to 0V of the ground voltage GND. This is because when the wire between the battery and the power device is long, the impedance of this wire becomes much higher than the ON resistance (several m—several tens mΩ) of the output MOS transistor, and if the load is shorted, most of the voltage of the battery is consumed by the wire.
Also the current is no longer consumed by the load, so the output current IOUT becomes abnormally heavy. Due to the over current, the output MOS transistor M110 heats up, and if the over current continues in this state, the output MOS transistor M110 breaks down by this heat. Therefore the output MOS transistor M110 must be turned OFF as quickly as possible.
However if the power supply voltage Vcc is extremely low, such as 0V, the over current protection circuit 104 cannot operate normally. For example, if the power supply voltage Vcc is the operating voltage of the detection circuit 105 or less, the over current cannot be detected, and if the power supply voltage Vcc is the threshold value of the MOS transistor M111 or less, the MOS transistor M111 does not turn ON, so the gate charges of the output MOS transistor M110 cannot be discharged. Therefore the output MOS transistor M110 cannot be turned OFF and the output MOS transistor M110 breaks down.
As a conventional semiconductor device having a circuit similar to FIG. 14 and FIG. 15, the one disclosed in Japanese Patent Application Laid-Open No. 2001-160746 is known.
As described above, in the case of a conventional semiconductor device, such as a power device, the output transistor cannot be turned OFF, and the output transistor breaks down if the power supply voltage is extremely low when an over current is generated.